Investigators have disclosed that the pilot of a tour helicopter was warned of the presence of a light aircraft moments before the two collided over New York, killing all nine on board.

The accident occurred at about noon yesterday after the aircraft, identified as a Piper PA-32, struck a Eurocopter AS350 which had departed from a helipad on West 30th Street, on the east side of the Hudson River.

She adds: "There was no response from the pilot. This is a [visual flight rules] corridor, you’re supposed to be alert and see and avoid other aircraft in the vicinity."

New York mayor Michael Bloomberg says the PA-32, with a pilot and two passengers, including a child, had taken off from Teterboro Airport in New Jersey, west of the river.

The helicopter had been carrying five Italian tourists and a pilot, he says. It had been among the fleet operated by Liberty Helicopters.

Bloomberg adds that the accident does not appear to have been survivable. "I think it's fair to say that this has changed from a rescue to a recovery mission," he states.

Preliminary indications suggest the PA-32 "ran into" the rear of the helicopter, he says, but cautions that the NTSB has yet to confirm details.

"There is some evidence from eyewitnesses that one of the wings of the airplane was severed, or separated, probably severed by the rotors of the helicopter," he says. Images captured immediately after the impact show the PA-32 descending with its starboard wing missing.

No information has been disclosed regarding the height at which the collision occurred. But weather data from Teterboro Airport at noon pointed to excellent visibility and minimal cloud in the vicinity.

The PA-32 was registered N71MC, says the NTSB. It is sending a 10-member team, headed by Robert Gretz, to investigate the collision.

Emergency teams have located wreckage from one of the aircraft - thought to be the helicopter - in the Hudson, on the New Jersey side opposite 14th Street, but Bloomberg says visibility in the river is poor.